1. Field of the Invention
This invention is concerned with conversion of the heavy end of crude petroleum and like source materials predominating in hydrocarbons and hydrocarbon derivatives such as tars (e.g. from tar sands) and the like. The conversion products are useful as fuels and as charge stocks for other conversion processes such as catalytic cracking, reforming etc.
With increasing demand for premium fuels such as motor gasoline, diesel fuel, jet fuel and furnace oils, the industry has increasingly been pressed to utilize poorer grade crude oils and to use greater proportion of the available crudes in manufacture of premium products. Many of the crudes contain metal compounds, sulfur compounds, nitrogen compounds and the highly condensed hydrocarbons sometimes called asphaltenes which lead to carbonaceous deposits in processing equipment and/or fuel nozzles and the like. These undesirable components are generally found in the higher boiling components of a crude petroleum and therefore tend to be concentrated during distillation of the crude into the higher boiling fractions, particularly the bottoms fractions of crude stills. Those bottoms are the unvaporized liquids, remaining after vaporization at atmospheric pressure or under vacuum. These are generally called "residual stocks" or simply "resids". This invention is concerned with catalytic conversion under hydrogen pressure to upgrade and convert the atmospheric and vacuum resids taken as bottoms from atmospheric and vacuum crude stills.
2. The Prior Art
A great many expedients have been proposed for dealing with the problems which arise in use of resids as fuels or as charge to such processes as catalytic cracking. Thermal conversions of resids produce large quantities of solid fuel (coke) and the pertinent processes are characterized as coking, of which two varieties are presently practiced commercially. In delayed coking, the feed is heated in a furnace and passed to large drums maintained at 780.degree. to 840.degree. F. During the long residence time at this temperature, the charge is converted to coke and distillate products taken off the top of the drum for recovery of "coker gasoline", "coker gas oil" and gas. The other coking process now in use employs a fluidized bed of coke in the form of small gradules at about 900.degree. to 1050.degree. F. The resid charge undergoes conversion on the surface of the coke particles during a residence time on the order of two minutes, depositing additional coke on the surfaces of particles in the fluidized bed. Coke particles are transferred to a bed fluidized by air to burn some of the coke at temperatures upwards of 1100.degree. F., thus heating the residual coke which is then returned to the coking vessel for conversion of additional charge.
These coking processes are known to induce extensive cracking of components which would be valuable for catalytic cracking charge, resulting in gasoline of lower octane number (from thermal cracking) than would be obtained by catalytic cracking of the same components. The gas oils produced are olefinic, containing significant amounts of diolefins which are prone to degradation to coke in furnace burners and on cracking catalysts. It is often desirable to treat the gas oils by expensive hydrogenation techniques before charging to catalytic cracking. Coking does reduce metals and Conradson Carbon but still leaves an inferior gas oil for charge to catalytic cracking.
Catalytic charge stock and fuels may also be prepared from resids by "deasphalting" in which an asphalt precipitant such as liquid propane is mixed with the oil. Metals and Conradson Carbon are drastically reduced but at low yield of deasphalted oil.
Solvent extractions and various other techniques have been proposed for preparation of FCC charge stock from resids. Solvent extraction, in common with propane deasphalting, functions by selection on chemical type, rejecting from the charge stock the aromatic compounds which can crack to yield high octane components of cracked naphtha. Low temperature, liquid phase sorption on catalytically inert silica gel is proposed by Shuman and Brace, OIL AND GAS JOURNAL, Apr. 16, 1953, Page 113.
Catalytic hydrotreating alone or in combination with hydrocracking is a recognized technique for improving resids. Contact of the resid with suitable catalysts at elevated temperature and under high hydrogen pressure results in reduction of sulfur, nitrogen, metals and Conradson Carbon (CC) content of the charge stock. Hydrotreating in the term applied here to operations over a catalyst of a hydrogenation metal on a support of low or negligible cracking activity. Metals, particularly nickel and vanadium are thereby split out of the complex molecules in which they occur and are deposited on the hydrotreating catalyst. Sulfur and nitrogren are converted to hydrogen sulfide and ammonia in hydrotreating and separated with a gas phase after condensation of the liquid hydrocarbons resulting from the treatment.
The hydrocracking catalysts are characterized by dual functions of a hydrogenation/dehydrogenation metal function associated with an acid cracking catalyst which may also serve as support for the metal, e.g., hydrogen form of ZSM-5. The hydrocracking operation removes sulfur, nitrogen and metals from the charge and also converts polycyclic compounds, including asphaltenes, by ring opening and hydrogenation.
In addition to its use in feed preparation, hydrotreating has also been applied in "finishing" of refinery products by desulfurization, saturation of olefins and the like. It has been proposed to combine the feed preparation and product finishing functions by blending intermediate gasoline, gas oils and like fuels with fresh crude. Suitable process flow diagrams for that purpose are described in U.S. Pat. No. 3,775,290 to Peterson et al. and U.S. Pat. No. 3,891,538 to Walkey. The latter at column 5, discusses the benefits of so recycling catalytic cycle oil boiling to 800.degree. F. and coker gas oil boiling to 900.degree. F. In addition, it may be speculated that the diluent effect of the recycled gas oils and the hydrogen donor capabilities of polycyclic compounds therein can be expected to improve hydrotreating of feed stocks which contain asphaltenes.
Nitrogen compounds are generally recognized as detrimental to the activity of acid catalysts such as those employed for cracking and hydrocracking. That principle is discussed in U.S. Pat. No. 3,694,345 in describing a hydrocracking catalyst which is effective in the presence or absence of nitrogen compounds. The process of U.S. Pat. No. 3,657,110 takes advantage of the deactivating effect on nitrogen compounds by introduction of high nitrogen feed along the length of a hydrocracker to moderate the exothermic reaction and aid in control of temperature.